The present invention relates to a process and device for the preparation of mixture comprising a solid phase and a liquid phase of a metal alloy, of the type of those which are utilized in the so-called "semiliquid molding processes".
It is known in metallurgy that the metal alloys have a thermal solidification range whose amplitude is characteristic of the alloy itself. Above the upper limit ("liquidus" point) of such range, the alloy is completely in the liquid state, whilst below the lower limit ("solidus" point) the alloy is in the solid state. In the solidification range two phases are present, a liquid phase and a solid phase, the respective quantity of which is a function of the temperature and the composition of the alloy.
In the conventional solidification conditions, the solid appears in a dendritic form; i.e. in the form of an arborescent skeleton, characterized by main branches, from which secundary, tertiary etc. branches extend perpendicularly.
Already at a solid fraction of 20% the dendrites which are present form a continuous arborescent skeleton which increases the viscosity values beyond the limits acceptable for a casting operation.
There are known processes by means of which it is possible to prepare a mixture containing a solid phase and a liquid phase of a metal material, which mixture, though having a rather high concentration of the solid phase, has the characteristic properties of the liquids, in particular a relatively low viscosity.
Some processes are directed to substantially provide a creep between the various particles of the mixture maintained in movement, so as to break, within certain limits, the dendritic interconnections which are formed during the solidification of the mixture, and to prevent the further growth of the dendrites; in this way, the dendritic fragments remain independent and tend to assume spheroidal shapes under the action of the continuous mechanical impacts.
The said creep, which is evaluated by means of the respective gradient, can be obtained in the interior both of a turbulent flow and a stationary fluid stream, i.e. a stream in which the various particles of the mixture move at predetermined speed depending on the position they have relative to the walls of the cavities they traverse.
A process is known, which consists in making an alloy traverse, in the melted condition, an axial channel annular in shape, which is obtained by means of a tubular container which is suitably cooled and in the interior of which a cylindrical entraining rotor rotates coaxially with the axis of the container itself; the particles of the forming mixture which are immediately adjacent the surface of the said rotor are made to rotate substantially at the same tangential speed as the surface of the rotor, whilst the particles situated at a greater distance from the said axis are entrained at a lower tangential speed which becomes substantially equal to zero for the particles which are in contact with the inner surface of the said container.
Therefore, a predetermined range of movement is established, in which occurs a gradual variation of the speed of the particles in the radial direction of the annular channel which is traversed by the mixture, with the result that a predetermined creep gradient between the particles is created; in this way, the dendritic bonds which tend to form in the mixture, as the mixture cools, break, thereby allowing to maintain the mixture relatively fluid, but still having a sufficiently high contents of a solid phase.
This process has some disadvantages. First of all, the time required for the treatment of a predetermined quantity of mixture is rather long; moreover, for a pre-established viscosity of the mixture obtained by means of the said process, the percentage of solid phase contained in it is not very high. These disadvantages are originated particularly by the fact the the creep gradient between the various flow lines of the mixture maintained in movement in the way described hereinabove is rather modest, with the result that the obtained action of breaking the dendritic bonds is not very efficient. In fact, in an annular channel, as is the channel which in the process described hereinabove is traversed by the mixture, it is not possible to obtain high creep gradients without giving rise to serious disadvantages; the increase of the gradient may be obtained only by reducing the radial dimension of the said channel (with the consequent reduction of the range of the flow) or by increasing the speed of the entrainment rotor (with the consequent increase of the power of the installation and the vibrations).
Furthermore, in the process described hereinabove, it is not possible to obtain a mixture having a high percentage of solid particles of small dimensions, because the thermal exchanges between the particles of the mixture are very low; in fact, the movements between the various lines of flow of the mixture (which depend only on the entraining action of the rotor) are rather modest, so that only small thermal exchanges between the particles are obtained.
Moreover, the process described hereinabove is not suitable for feeding in a continuous manner the mixture to an integral forming plant (for example a molding plant, a pressure die-casting plant), because some stages of the process cannot be carried out without interruption.
Finally, the performance of the process described hereinabove requires the use of a device whose structure is not simple and whose overall dimensions are rather large, owing both to the presence of the entraining member mentioned hereinabove and, above all, the relative rotation which has to be obtained between this latter and the container in which it is located; in fact, support means are necessary for the said element, as well as a suitable source of movement and suitable transmission means.